This invention relates generally to ink jet printing systems, and more particularly to a charge plate and method of fabrication of a charge plate for use in an ink jet printing system.
In ink jet printers, such as the systems shown by Sweet et al., U.S. Pat. No. 3,373,437, which print on a moving web with uncharged ink drops while deflecting and catching charged drops, charge electrodes have performed the critical function of selectively charging the ink drops. The drops of ink are formed from fluid filaments which emerge from orifices in an orifice plate communicating with an ink fluid reservoir in which electrically conductive ink is maintained under pressure. By stimulating the orifice plate or the fluid in the reservoir, the fluid filaments emerging from the orifice plate are caused to break up into uniformly sized and spaced drops. As each drop breaks off from a fluid filament, it is selectively charged or left uncharged in a predetermined pattern by an associated charge electrode. The drops then pass through an electrostatic deflection field with the charged drops being deflected into and caught by a drop catcher. The uncharged drops remain undeflected and continue past the deflection field to impact on a moving print web in a readable pattern.
Charge electrodes previously used in the art have comprised an electrically conductive material coated or otherwise deposited onto a nonconductive substrate. Many such electrodes have partially or wholly surrounded the corresponding ink jet stream and extended uniformly along the stream for a distance of at least several drop diameters. Because of the tendency of the ink drops to break off from the filaments at different points, the electric field produced by the charge electrode must be uniform along the length of the ink filaments so that drops may be properly charged without regard to their exact breakoff points. Early patents to Loughren, U.S. Pat. No. 3,404,221, and Sweet et al., U.S. Pat. No. 3,373,437, utilized cylindrically shaped hollow rings or tubes or U-shaped channels as charge electrodes. However, the accurate placement of the tubes or channels into a support structure and electrically connecting such devices to a signal source was both difficult and time consuming, especially in multi-jet systems utilizing hundreds of individual streams of ink drops spaced only fractions of millimeters apart.
Several workers in the art have attempted to reduce the difficulty and expense of forming charge electrodes. For example, Beam et al., U.S. Pat. No. 3,586,907, shows a charge ring plate with a series of holes therein and having a coating of an electrically conductive material surrounding each hole and extending along the walls of the hole forming charge rings. Electrical lead lines are also plated onto the surface of the charge plate and extend from each charge ring to a connection point. The techniques involved in plating the walls of the holes to obtain a continuous and uniform coating are complex and involve plating in several dimensions. Likewise, coating the U-shaped channels shown in Culp, U.S. Pat. No. 3,618,858, with an electrically conductive material also involves plating in several dimensions.
Another example of forming charge electrodes is shown by Robertson, U.S. Pat. Nos. 3,604,980 and 3,656,171, in which a dielectric planar surface has plated thereon a series of strips of electrically conductive material, each connected to a charging signal source. Robertson differs from previous prior art charge electrodes in that the conductive strips do not surround or partially surround the drop streams. However, the formation of the conductive strips still involves plating in several dimensions.
Kenworthy, U.S. Pat. No. 4,223,321, also, like Robertson, utilizes a flat face charge plate having individual electrodes formed by filling spaced apart grooves in the face of a nonconductive substrate with electrically conductive solder and lapping off excess material. The electrical connections to each electrode are formed by masking and vapor deposition techniques. However, the shrinkage of the solder during solidification and the complexity of forming the electrical connections still presents problems in obtaining a uniform and reliable charge plate.
Accordingly, the need still exists in the art for a relatively simple method for forming a multiplicity of uniformly spaced charge electrodes to form a charge plate for an ink jet printing system.